Drought

Long-term drying trends, 1900-2008

While climate change has increased precipitation in some areas, in other regions it has contributed to drought.1 Though there are a number of factors that drive drought, such conditions are apt to develop in regions that lack rain; drought is also greatly intensified by increased evaporation from soil and vegetation associated with warming.2 Very dry areas across the globe have doubled in extent since the 1970s.3 In particular, a long-term drying trend (from 1900 to 2008) persists in Africa, East and South Asia, eastern Australia, southern Europe, northern South America, most of Alaska, and western Canada.4

The global increase in drier, hotter areas and the trend in which dry areas are becoming drier can both be traced to the human influence.5 Drying trends have been observed in both the Northern and Southern hemispheres since the 1950s.6 These trends cannot be explained by natural variations but do fit well with climate model expectations for global warming.7 In particular, greenhouse gas emissions have contributed significantly to recent drying by driving warming over land and ocean.8

The increase in drought is caused by many factors: shortfalls in precipitation; earlier snow melt; a shift away from light and moderate rains towards short, heavy precipitation events; and increased evaporation from soil and vegetation due to higher atmospheric temperatures, all of which have been driven at least in part by climate change. Increased heating leads to greater evaporation of moisture from land, thereby increasing the intensity and duration of drought.9

Individual droughts have been linked to climate change, such as the drought that hit central India in 2008 when the north-south pattern of precipitation was disrupted by unusual weather driven by abnormally high sea surface temperatures due in part to global warming.10

Wet and Dry Extremes Both Increasing Across the Globe

The increase in drought is caused by many factors: shortfalls in precipitation, early snow melt, a shift away from light and moderate rains towards short, heavy precipitation events, and increased evaporation due to higher temperatures, all of which have been driven in part by climate change. Increased heating leads to greater evaporation of moisture from land, thereby increasing the intensity and duration of drought.11

Alterations in atmospheric circulation have contributed to the distinctive pattern in global precipitation changes in recent years whereby the subtropics and much of the tropics (with the exception of the monsoon trough) have become drier.12 Rapid warming since the late 1970s has both evaporated large amounts of moisture from the land into the atmosphere and altered atmospheric circulation patterns, contributing to the drying over land.13

Individual droughts have been linked to climate change, such as the drought that hit central India in 2008 when the north-south pattern of precipitation was disrupted by unusual weather driven by abnormally high sea surface temperatures linked to global warming.14

Spring Snowmelt Shifting to Earlier in the Year

Spring Snowmelt Shifting to Earlier in the Year

Date of onset of spring runoff pulse. Redish-brown circles indicate significant trends toward onsets more than 20 days earlier. Lighter circles indicate less advance of the onset. Blue circles indicate later onset. The changes depend on a numer of factors in addition to temperature, including altitude and timing of snowfall.

Karl et al. 2009

Rising temperatures have also led to earlier melting of the snowpack in the western United States, more than 20 days earlier in some locations.15 Early snow melt, along with an increased tendency for precipitation to fall as rain rather than snow, can be an important contributor to drought in regions that count on snowpack to supply water, such as the western U.S. and Canada. A recent study of water cycle changes in the western U.S. attributes to human influence up to 60% of observed climate-related trends in river flow, winter air temperature, and snow pack in the region over the 1950–1999 period.16